Developments in combustion engine technology have shown that compression ignition engines, frequently referred to as diesel-cycle engines, can be fuelled by gaseous fuels instead of diesel without sacrifices in performance or efficiency. Examples of such fuels include natural gas, methane, propane, ethane, gaseous combustible hydrocarbon derivatives and hydrogen. Substituting diesel with such gaseous fuels generally results in cost, availability and emissions benefits over diesel.
One challenge in using gaseous fuels for such applications, however, is that it is difficult to store gaseous fuels as efficiently as liquid fuels. This is an important consideration where space for storage is limited. For example, on-board fueling systems for natural gas vehicles benefit when those systems can be accommodated in the limited space available freeing up transport capacity.
Natural gas and other gaseous fuels can be stored in tanks either as compressed gas (CNG in the case of natural gas), or cryogenically as a liquid (LNG in the case of liquefied natural gas). The advantage of LNG over CNG is that its energy density is much higher than an equivalent volume of CNG. Natural gas stored as LNG allows for more fuel to be stored per unit volume.
For the purposes of this application, cryogenic fuels include those liquids fuels that boil at temperatures at or below −100 C. under atmospheric pressures. An example of such fuel is LNG. Note, while the present invention is discussed in regards to LNG, it is equally as applicable to other cold or cryogenic fuels or gases generally. This would be understood by a person skilled in the art. By way of example, the disclosed tank accommodates other hydrocarbons such as methane, ethane, propane and hydrocarbon derivatives. Further fuels and gases such as hydrogen, helium, nitrogen and oxygen all benefit as cryogens to the present invention.
While, as mentioned above, there are special economies to utilizing LNG, cryogenic storage presents its own challenges. One of the challenges of LNG is that, in many applications, once delivered into a holding tank, it needs extra space in which to expand if the LNG warms.
While the main concern to be addressed in this application is LNG expansion, LNG may, in some applications, need to be stored at cryogenic temperatures over extended periods of time. Excessive heat leakage into a cryogenic tank, as well as causing the LNG itself to expand, will cause the cryogenic liquid to boil. Eventually, with continued heat leakage, LNG will boil or evaporate resulting in excessive stresses on the storage tank caused by pressure build-up. A way of dealing with pressure build-up associated with boiling or evaporation (or, for that matter, LNG expansion) is to provide a relief valve in the inner vessel to vent off gas after the pressure in the vessel has reached a pre-determined limit. However, for a variety of reasons, it is undesirable to routinely vent natural gas into the atmosphere. For example, methane, the major component of natural gas, is a greenhouse gas. Also, venting fuel is uneconomical as this releases unused fuel into the atmosphere. Therefore, an alternative solution to venting is desirable.
One further way of limiting the amount the LNG expands or the amount of LNG to boil over time, is to ensure that the storage tank is well insulated minimizing the amount of heat allowed to leak into the tank. By way of example, an evacuated space can be used to separate and insulate an inner storage vessel where the cryogen is stored from an outer jacket exposed to the ambient environment. However, regardless of the effectiveness of the insulation used to prevent heat leakage into the cryogenic fluid or liquid, some heat paths will exist between the outer jacket and the storage vessel. For example incomplete evacuation of the space disposed between the outer jacket and inner vessel may cause heat leakage. Heat paths may also arise from support members within the evacuated space provided to suspend the inner vessel within the outer jacket. Conduits or lines into the cryogen space used, by way of example, to fill, empty, and vent the cryogen space within the inner vessel, also introduce heat paths. In any event, the inner vessel cannot be completely insulated.
A means of dealing with LNG expansion is to provide an ullage space within the inner vessel. An ullage space can also be used to accommodate any boiling or evaporation of LNG in some applications where LNG is stored after a vessel is filled as would be understood by a person skilled in the art. For most applications, however, an ullage space is provided to deal with liquid expansion. For the purposes of this disclosure, while reference will be made to accommodating LNG expansion, the ullage space can be adapted to accommodate LNG expansion or boiling or evaporation, as would be understood by a person skilled in the art.
An ullage space is a space within the tank for the LNG to expand into. One problem with this solution is that it is difficult to leave an adequate space during filling. In other words, refueling must be stopped at some pre-determined point prior to the storage tank reaching liquid full. Ideally, the ullage space should be large enough to allow for LNG expansion yet small enough to maximize the amount of cryogen that can be held in the inner vessel and, thereby, maximize the time between refueling. As noted above, this is valuable in natural gas vehicle operations where fuel systems attempt to maximize the volume they can store within the limited space available on a vehicle while minimizing the space utilized to store that fuel.
A variety of means have been developed to determine a fill point that leaves an adequate ullage space.
Visual fill lines may not provide the level of accuracy required. Also, given the double vessel structure of many tanks, it is not easy to provide a sight port through to the inner vessel.
Stop mechanisms such as shut-off floats or valves require mechanical parts within the inner vessel. This introduces into the storage tank a mechanical failure point that is subjected to wear during and between each fill. The introduction of such a failure point may, in many cases, be the failure point that determines the life of the tank. Access to the interior of the tank, and therefore, to mechanical parts within the tank is difficult if not impossible due to the need to thermally isolate the inner tank from the outside environment. Therefore, repair of a mechanical part is not practical in many cases. As such, failure of a mechanical stop mechanism generally requires replacement of the storage tank.
Further, such a stop mechanism needs a means of communicating with the filling pump directly or indirectly so that it is able to shut-off the pump once the ullage space has been provided. Such a means of communication may introduce another heat path into the cryogen space within the inner vessel.
A further alternative to providing an ullage space is to introduce an ullage vessel in communication with the inner vessel. This vessel would include a valve or restrictive opening dimensioned to allow gas to flow into the ullage space during filling but restrict flow of liquid. Once the space outside the ullage vessel is full, cryogen will then be forced into the ullage space within the ullage vessel. Due to the dimensioning of the ullage opening in the ullage vessel or the nature of a valve disposed in the ullage vessel, the resistance to flow of cryogenic liquid into the ullage space once the cryogen space is liquid full will create a pressure rise that can be detected by the fill pump or operator. When detected, the fill pump will shut off. Some of the gas within the ullage space will be cooled and condense out during filling as the cryogen space fills cooling the ullage vessel.
This system, however, will, generally, leave a quantity of gas within the ullage space at the end of filling the cryogen space that is equal to or greater than the quantity in the ullage space initially. While some of the gas present prior to filling will be condensed to liquid by the cooler cryogen environment, this will, in many cases, be a relatively small amount of the total residual gas initially present. Also, some of the cryogen provide may evaporate into this space during filling. The volume of gas initially present within the ullage space plus any additional evaporated gas from the cryogen added to the cryogen space should, therefore, be taken into consideration when determining the desired volume of the ullage space.
Further, this method of providing an ullage space presents problems in the case where the cryogen tank has not been emptied between fills. After a refueling, the ullage vessel will allow LNG to expand into the ullage space due to the processes described above. Also, gravity will frequently force, over time, LNG into the ullage vessel through the restrictive opening. As such, depending on the orientation of the opening in the ullage vessel and the level of the cryogen tank generally, the ullage vessel will be at least partially filled with LNG until the LNG in the cryogen space is drawn down below the ullage space opening allowing gravity to force any LNG in the ullage space to flow out of this space into the cryogen space. In many cases, however, it is desirable to refuel prior to a point where the LNG tank is completely empty. In the prior art design, however, initial refueling is into a tank with a partially filled ullage space. As such, the ullage space may not inadequately allow for LNG expansion upon completion of refueling.
The present invention addresses the problems in the prior art noted above.